Research on the handling of liquid hydrogen in space
ZARM scientists are studying the handling of cryogenic propellants in the tanks of rocket upper stages and orbital fuel depots and were able to investigate the growth of individual vapor bubbles in liquid hydrogen under microgravity conditions.
Scientists are studying the handling of cryogenic propellants in the tanks of rocket upper stages and orbital fuel depots
In the Bremen Drop Tower at the Center for Applied Space Technology and Microgravity (ZARM) at the University of Bremen, researchers were able to observe the growth of individual vapor bubbles in liquid hydrogen under microgravity conditions. The series of experiments provided valuable data for a better understanding of the fluid dynamics and thermodynamic processes associated with cryogenic fuels under reduced gravity. The research results, published on March 12, 2026, in the journal Cryogenics, are of particular interest for the storage of liquid hydrogen in orbit, e.g., in a rocket upper stage or a future fuel depot in space.
Hydrogen and oxygen are one of the most efficient fuel combinations for modern space propulsion systems. To create a volume- and mass-optimized spacecraft tank, the cryogenic propellants are cooled until they reach a liquid state. In this state, the cryogenic liquid has a higher density than in its gaseous state and can be stored in smaller, thinner-walled tanks at near-ambient pressure. However, handling cryogenic liquids under reduced gravity poses major challenges for industry and the scientific community: even small local superheating of the tank wall or the lines can cause vapor bubbles to form. These bubbles originate at so-called nucleation sites on the liquid-wetted inner surface of the tank.
This process is known as nucleate boiling and can significantly affect the thermodynamic state of the fuel in the tank: The formation and growth of the bubbles result in a mixture of liquid and gas, which increases the pressure in the tank and changes the quality of the wet steam. There is also a risk that ingested vapor bubbles may pass through the tank outlet next to the pure liquid and affect the engine operation.
Discoveries from the Bremen Drop Tower
In the Bremen drop tower, researchers were able to specifically induce and observe the formation and growth of individual hydrogen vapor bubbles. To do this, an artificial nucleation site in the form of a small cavity in the material was heated. Since the experiments were conducted under reduced gravity, the vapor bubble remained at its point of formation and was not displaced by buoyancy (as it would be under Earth’s gravity).
The recorded experimental images and measurement data enable quantification of the bubble growth process. A heat flux of just 80 milliwatts dissipated by the heater was sufficient to activate the artificial nucleation site. Within 3.6 seconds, a vapor bubble subsequently grew to a diameter of about four millimeters. In conjunction with the thermal boundary conditions, the experimental results provide important data for further development of numerical models of the boiling behavior of liquid hydrogen in space.
“Our experimental results contribute significantly to a better understanding of the thermodynamic processes in cryogenic fuel systems,” says André Pingel, head of the “Multiphase Flows” research group.
The experiments were conducted as part of the successfully completed project “Cavitation and Boiling of Methane and Hydrogen” (KASIMOFF 2). The project was funded by the Federal Ministry for Economic Affairs and Climate Action (BMWK) through the German Aerospace Center (DLR e.V.) under grant number 50RL2290.
Further Information
Link to the publication: https://www.sciencedirect.com/science/article/pii/S0011227526000536?via%3Dihub
Scientific contact:
Dr.-Ing. André Pingel
andre.pingel(at)zarm.uni-bremen(dot)de
Press contact:
Jasmin Plättner
jasmin.plaettner(at)zarm.uni-bremen(dot)de
+49 0421 218-57753
